Exercise treadmills provide ordinary individuals with a means to maintain fitness, those recovering from injuries a means to rehabilitate themselves, and cardiologists or other health care professionals with a diagnostic instrument to measure fitness. The most common type of treadmill is driven by a DC motor that allows the user to directly and accurately control the speed of the belt. The use of a DC motor generally provides for a smaller, mechanically less complicated and less expensive belt driver mechanism. Unfortunately, DC motors are not as reliable as AC motors in treadmill applications.
A few exercise treadmills have been provided with variable speed AC motor drives. Typically, these AC motor driven treadmills are provided with a transmission for varying the speed of the walking belt. The transmission permits the AC motor to revolve at a constant angular velocity while the gearing in the transmission serves to vary the speed of the walking belt. In such a system, the transmission will normally leave the speed setting of the belt at whatever speed it was rotating prior to being shut off. One advantage of the use of a transmission with an AC motor is that it tends to insulate the motor from sudden inertial changes such as heel strike forces by heavy users.
Treadmills using DC motors adjust the speed of both the motor and the belt by directly controlling electrical power input to the motor. DC treadmills start at a zero velocity and increase speed with more power input into the motor and likewise decrease speed with less power input into the motor. DC motors use various types of controllers to vary the power to the motor, most commonly SCR type controllers. Some types of DC treadmills, such as one manufactured by Biodex, use four-quadrant, pulse width modulation type controllers. In any case, DC motor treadmills require circuitry capable of rectifying the standard 60 Hz AC power into a variable level direct current to power the DC motor.
Speed regulation in the treadmill industry refers to the ability of a motor controller to maintain a constant speed even when the load on the motor changes. The load on the treadmill motor would change when, for example, it is set at 6 miles per hour and the user attempts to accelerate or slow down. Thus, any treadmill motor controller must be able to change the power input to the electric motor very quickly so as to counter sudden load changes.
Variable speed direct drive AC motors have heretofore been available in a number of different industries for a variety of applications. Frequency driven AC motors are driven by an invertor that varies the voltage and the frequency of the power delivered to the AC induction (non-synchronous) motor to thereby control its speed.
On the other hand, one type of treadmill currently in use, manufactured by the assignee of the present invention, includes a walking belt driven by a synchronous AC motor. A synchronous motor is one that is typically maintained at constant rotational speed. To adjust the speed of the walking belt, the motor is connected through a variable transmission consisting of two sets of adjustable sheave pulleys. As indicated above, the use of a synchronous AC motor-transmission combination typifies the limited use of AC motors in the treadmill industry.
Heretofore, AC motor drives in general lacked practical or commercial viability in the exercise treadmill industry because of their excessive weight, expense, complexity, and the lack of available direct drive control mechanisms. In addition, there are a number of speed and acceleration requirements specific to the treadmill industry that have prevented the incorporation of AC motors. Specifically, exercise treadmills must start slowly from a dead stop and gradually reach their final set speed. That is, if the speed control unit of the treadmill is left at a setting of, for example, 8 mph and the treadmill is suddenly turned on, the preferred treadmill should be able to slowly reach 8 mph. If it did not, the sudden speed increase from 0-8 mph may cause the user to stumble. Other unique problems associated with human exercise treadmills have limited the space for the drive components of the treadmill. Aesthetics, convenience, and practicality favor a small hood structure forward of the walking belt. Weight, width and height limitations are necessary because such treadmills are frequently moved in and out of rooms, occasionally through narrow doorways.
Thus, an AC motor driven treadmill, to be commercially viable should be constructed in a small, lightweight, simple, inexpensive unit capable of starting up and shutting down slowly. Further, it must be competitive with the available DC drive technology and products. The AC motor should be associated with a proper controller/invertor that can match load and belt speeds. Finally, the AC motor drive must be durable and quiet so as to last long and not disturb the user.
In the present invention, applicant provides a novel direct drive, variable speed, AC motor powered exercise treadmill, controlled through a variable frequency/variable voltage motor controller and powered by a standard AC external power source. A treadmill control/display generates digital output signals to a relay/control board which provides an analog signal to the motor controller which, in turn controls the speed of the AC motor. The AC motor directly drives the treadmill belt. The motor controller provides a signal input to the AC motor as a function of the input signal from the control board. A speed sensor capable of detecting the speed of the AC motor, and thus the treadmill belt, is connected to the control/display and thus provides a closed feedback loop for maintaining the AC motor at preselected speed or to vary the speed in response to preselected time variable commands.